In recent years, from the viewpoint of global warming, to suppress the exhaust of room temperature effect gases (CO2, NOx, and SOx), progress has been made in development of technology which uses hydrogen as energy. In the past, when storing hydrogen as high pressure hydrogen gas, thick-walled Cr—Mo steel tanks have been filled with hydrogen gas up to about pressures of about 40 MPa.
However, tanks made of such Cr—Mo steel fall in fatigue strength due to fluctuations in inside pressure and penetration by hydrogen due to repeated filling and release of high pressure hydrogen, so the wall thickness must be made 30 mm or so. The weight therefore builds up. For this reason, the increase in weight and increase in size of the equipment become serious issues.
The existing JIS standard SUS316-based austenite stainless steel (hereinafter referred to as “SUS316 steel”) has a hydrogen embrittlement resistance under a high pressure hydrogen gas environment which is better than other steel for structural use, for example, carbon steel, including the above Cr—Mo steel, and JIS standard SUS304-based austenite stainless steel (hereinafter referred to as “SUS304 steel”), so is being used for piping materials or high pressure hydrogen fuel tank liners of fuel cell vehicles as well.
SUS316 steel is stainless steel which contains expensive Ni: 10% or more and Mo: 2% or more. For this reason, SUS316 steel has major problems in general applicability and economy (cost).
Further, to store and transport a large amount of hydrogen gas, making the pressure of the hydrogen gas a high pressure of over 40 MPa and making active use of liquid hydrogen may be mentioned. Regarding the increase in pressure, for example, to use piping made of SUS316 steel in an over 40 MPa high pressure hydrogen gas environment, the point has been raised that it would be necessary to increase the currently 3 mm wall thickness piping to over 6 mm thickness or the piping would not be able to withstand use strength-wise.
For ultralow temperature use of liquid hydrogen, in the past, austenitic SUS304 steel or SUS316 steel has been used. For liquid hydrogen containers as well, low temperature hydrogen gas embrittlement has to be considered at the top layer part where the liquid hydrogen becomes vapor, so it is preferable to use SUS316 which is excellent in hydrogen embrittlement resistance.
Further, in recent years, in advance of the introduction of fuel cell vehicles, progress has been made in official building of prototypes of hydrogen stations and running of proving tests. Hydrogen stations which store large amounts of hydrogen as liquid hydrogen and which can raise the liquid hydrogen in pressure and supply it as over 70 MPa high pressure hydrogen gas are also in the proving stage. As the world moves to commercial use and popularization of such hydrogen stations, the need for an inexpensive metal material reduced in Ni and Mo and an inexpensive and high strength metal material which are able to be used in both hydrogen environments of high pressure hydrogen gas and liquid hydrogen has been becoming stronger.
In the past, high nitrogen-content austenitic stainless steel has been known as stainless steel for high pressure hydrogen gas use which is raised in material strength.
For example, PLT 1 discloses stainless steel for high pressure hydrogen gas use which contains N: 0.1 to 0.5%, Cr: 22 to 30%, Ni: 17 to 30%, Mn: 3 to 30%, and any of V, Ti, Zr, and Hf and which satisfies 5Cr+3.4Mn≦500N and containers and equipment comprised of that steel.
Furthermore, PLT 2 discloses stainless steel for high pressure hydrogen gas use which contains N: 0.1 to 0.5%, Cr: 15 to 22%, Ni: 5 to 20%, Mn: 7 to 30%, and any of V, Ti, Zr, and Hf and which satisfies 2.5Cr+3.4Mn≦300N and containers and equipment comprised of that steel.
The stainless steels which are disclosed in these PLT 1 and PLT 2 are directed to higher Cr and higher Ni compared with SUS316 steel. In the stainless steel which is disclosed in PLT 2 with a relatively small content of alloy elements as well, substantially the amount of Cr is over 17%, the amount of N is over 0.25%, and Ni, Mn, Mo, Nb, etc. are contained making it high alloy steel.
PLT 3 discloses a pressure vessel and pipe for piping use which are excellent in hydrogen environment embrittlement resistance and stress corrosion cracking resistance and which can be used for 70 MPa or more high pressure hydrogen gas without depending on a larger wall thickness and larger diameter. The steel which is used for these pressure vessel and pipe for piping use is comprised of a composition of ingredients of Cr: 15 to 20%, Ni: 8 to 17%, Si: 1.3 to 3.5%, Mn: 3.5% or less, and N: 0.2% or less.
PLT 4 discloses austenitic stainless steel welded pipe which is suitable for transport of 40 MPa or so high pressure hydrogen which is made of stainless steel containing Cr: 14 to 28%, Ni: 6 to 20%, Si: 4% or less, Mn: 3% or less, and N, 0.25% or less.
The stainless steels which are disclosed in PLT 3 and PLT 4 feature addition of Si and reduction of Mn and contain Ni in amounts of substantially 9 to 15% or about the same as or more than SUS316 steel.
The inventors proposed in PLT 5 austenitic high Mn stainless steel which has workability which enables cold working, deep drawing, and other press forming by a high working rate and is maintained in nonmagnetic property without formation of strain-induced martensite even after working. This stainless steel has trace amounts of Ni: 6% or less and Mo: 0.3% or more added and is remarkably superior in economy compared with SUS316 steel.
Furthermore, the inventors proposed in PLT 6 an austenitic high Mn stainless steel for high pressure hydrogen gas use which is intended for application in low temperature hydrogen gas environments and is inexpensive or both inexpensive and high in strength. This austenitic high Mn stainless steel pursues thorough reduction of alloying and as a result recommends addition of Cr: less than 15%, Ni: 6% or less, N: 0.01 to 0.4%, and a 0.35% trace amount of Mo and defines a parameter Md30 of austenite stability of −120 to 20 in range.
However, this austenitic high Mn stainless steel not only does not consider high pressure hydrogen gas, but also does not consider adaptation to a liquid hydrogen environment. The material properties under the ultralow temperature of liquid hydrogen are unknown.
Therefore, as explained above, no inexpensive stainless steel or inexpensive and high strength stainless steel which can be used in both hydrogen environments of over 40 MPa high pressure hydrogen gas and liquid hydrogen has yet appeared.